If you’d asked me ten years ago what homology was, I probably would’ve answered, “The study of… homos…?” I had no idea, because I’d never bothered to explore any evidence in favor of evolution. But it was evidence from homology — the study of similarities in animals due (allegedly) to common descent — that first prompted me to have second thoughts about evolution.
While Darwin may have been clued into evolution by birds, my personal journey began with man’s best friend: dogs. This was because, ironically, most creationists openly accept that all dog breeds originated from a single pair of dogs, and that this “kind” of dog was aboard Noah’s ark.
While you wouldn’t think creationists would be too eager to believe in such a rapid “micro-evolution” of dogs, this kind of variation is well documented, and it also helps to reduce the number of animals that would’ve been required to be aboard the ark, helping the ark story seem all the more plausible.
Even though today there are more than 350 variations of dogs, Noah only had to take one pair of dogs on the ark. Over time, during the 4,400 years since the flood, this one kind of dogs, through normal genetic variation, has given us the different varieties of dogs we see today. This isn’t evolution, it is normal genetic variation.
Geneticists agree that all dogs descended from a single kind, and specify it was some type of wolf ancestor. All dog breeds carry the genes of their wolf ancestor, and even continue to exhibit a few wolf-like traits (e.g. howling).
It was my curiosity about these rapid “genetic variations” that drove me to examine a few canine skulls, and I found the diversity among dogs to be quite extraordinary. It was amazing to think that from one type of dog, you could actually derive hundreds of very different skull shapes and sizes, and after just a few centuries of artificial selection.
Granted, these dogs were still dogs, but their ability to change over time did suggest that our skeletons were highly malleable. I began to wonder… if a wolf can become a pug or a chihuahua after just a few thousand years, how much more might an animal change if given a few million?
A visit to the California Academy of Sciences
With these dog skulls on my mind, I paid a visit to the California Academy of Sciences in San Francisco, and it further changed the way I perceived evolution.
It started with the T-rex skeleton. In my mind, humans had absolutely nothing in common with tyrannosaurs. And yet… I couldn’t deny the existence of similarities in our body plans: we both had vertebrae and a rib cage, arms, legs, knees, shoulder blades, hands, feet, and a skull with two eyes, a nose and mouth all arranged in the same general order.
Next came the museum’s blue whale. Again, its skeleton was also different from our own, but it too had ribs, a spine, and… arms. Arms? What was a whale doing with arms? I’m embarrassed to admit that I’d never really noticed how similar a whale’s arms are to our own… the fingers, the wrist bones, the radius and ulna, the humerus… even the shoulder blades were located in the same position.
Again I wondered… why would God create flippers out of arms and hands if he didn’t have to? It seemed like an odd choice. It was as if God had already testified by every fish in the sea that fish-like pectoral fins were the de facto standard for ocean dwellers, yet when it came to the whale, he decided, “Eh, I guess arms could work too.”
Later that day I perused their display of human skulls. As any good creationist knows, there are no “transitional” forms, there are only apes and humans with minor variations within each kind. But clearly, these skulls suggested that we humans could also undergo changes to our skulls, just as the dogs had done.
Lastly, I came across something I found even more compelling: the skeleton of a ring-tailed lemur.
On the outside, lemurs look very little like us, but under the skin, they are surprisingly similar. The arms, the legs, the shoulder blades, the pelvis, the ribs, and especially the tiny hands and feet… it’s all there, just shaped slightly different. Admittedly the skull was pretty different, but from what I’d seen in dogs and humans, I knew that skull shapes could change dramatically.
Upon returning home, I pondered these similarities, and began looking at pictures of other primate skeletons. There were some, like the spider monkey (below), that already had a skull that was much more human-esque than the lemur’s. In fact, just drop the tail and stand the spider monkey upright, and it appears almost human.
I wondered more… if dog skulls can change so drastically, and monkey and human skeletons were already similar, how can I argue that it would be impossible for (at the very least) the skeletal system of a primate to take on the shape of a human? This wouldn’t make it human, but it would have a nearly identical infrastructure. Just upgrade the brain, and you’re pretty much there.
Since then, I’ve learned what I’d inadvertently “discovered” was a branch of evidence for evolution known as homology. The great thing about homology is that you don’t need to be an expert in comparative anatomy to see these similarities. You can look at the forelimb of a human, a lizard, a cat, a whale, a bat, a frog, or a bird and see the same bones performing different functions.
And it doesn’t end with forelimbs, you can take any two mammals or reptiles and find equivalent structures throughout each.
Forelimbs of people, porpoises, bats and horses provide the classic example of homology in most textbooks. They look different, and do different things, but are built of the same bones. No engineer, starting from scratch each time, would have built such disparate structures from the same parts.
~ Stephen J. Gould, The Panda’s Thumb: More Reflections in Natural History, 1980
Creationist explanations for homologous relationships
So why do different species share such striking similarities? According to biologist and creationist Jerry Bergman, God created “systems” of animals, like mammals and reptiles, and then modified them so they could survive on land, in the air, and in the sea:
The many similarities that exist among members of the animal kingdom is the result of the fact that a single designer created the basic kinds of living ‘systems’, then specially modified each type of life to enable it to survive in its unique environmental niche. Examples of major environments for which organisms must be designed include the air, ground and water.
~ Jerry Bergman, Ph.D., Biology
Assuming this is true, it’s curious that God should equip some animals for one niche, and then place them in another. Why equip flightless birds with wings for the air niche, but then place them in the ground niche? Or why modify mammals and reptiles (e.g. whales, dolphins, manatees, sea turtles, etc.) to live in the water niche, but then equip them with the lungs of an animal intended for the ground niche? With evolution we expect such nonsense, but with a designer?
Also, if God did adapt these animals to survive in these niches, why then have so many failed to survive in the niches God prepared them for? Did God design them to go extinct? Was he unaware of the environmental changes they would surely face?
And finally, a full transition from sea-to-land and back again is time-consuming. If evolution did occur, we should expect to find very few mammals and reptiles occupying the open ocean compared to all other types of marine life, and this is the kind of thing we observe. It would’ve been no trouble for God to populate our oceans with mammals or reptiles, but he didn’t, once again making his creation frustratingly consistent with evolution.
While evolutionists focus on similarities, creationists focus on differences, often claiming that some of these differences are impossible to explain via evolution.
Even similarities among somewhat similar creatures reveal that the differences are more important than the similarities.
Answers in Genesis, March 1, 1992
Creationists add that no fossils exist that show a transition from any one kind of animal to another. Evolutionists object, and point to fossils like Tiktaalik (fish to land animal), Tritheledontidae (reptile to mammal), Archaeopteryx (dinosaur to bird), Ambulocetus (land mammal to whale), Darwinius masillae (prosimian and simian), Australopithecus (ape-like ancestors to human), and so on, but creationists counter by saying that because no one observed these transitions, God may have created them in the form they were all found in.
Homologous vs. Analogous similarities
I should also note that not all homologous similarities stem from common decent. For example, marsupial moles, wolves, and anteaters resemble their unrelated placental equivalents, but did not descend from them.
Creationists say these similarities exist because God (who must’ve been running out of ideas) remade similar animals in different classes. Evolutionists say these analogous similarities are the result of convergent evolution, and that these animals developed similar features because it helped them to accomplish similar tasks (e.g. long noses are beneficial to all ant eaters).
An interesting example of homology vs. analogy can be seen in whales, dolphins, and manatees. Whales and dolphins share homologous similarities with each other, but analogous similarities with manatees.
The distinctive homologous traits that suggest whales and dolphins had a common ancestor include the shape of their flukes, the bones in their flippers, and their trademark blowholes. Meanwhile, the manatee is also believed to have returned to the sea (or was modified for the sea), but seems to share only analogous similarities with whales and dolphins.
All three have traded in their hind legs for flukes, but the manatee’s fluke has a distinctively different shape. All three have also traded in arms for flippers, but the bones in the manatee’s flippers remain much more hand-like in appearance. And while whales and dolphins share the same tell-tale blowhole, the manatee has retained his nose.
Was the blowhole not good enough for God to reuse on the humble manatee? If God is modifying mammals for the water niche, why not equip them with the same equipment? If he’s not going to give the manatee gills, the least he could do is give him a blowhole.
In the same way I can see the familial relationships in dogs, I look at the blowholes on whales and dolphins and see a familial relationship. Likewise, I can look at the trademark pouches on all the marsupials stranded on Australia and infer that they too must all be related. And if all of those animals are related, it’s not too difficult to look at the similar skeletal structures of monkeys, apes, and humans and infer a similar familial relationship.
If evolution did occur, we would reasonably expect to see these kinds of relationships, and we do! Homology by descent is not only consistent with the evolutionary model, it is also easily falsifiable: if all animals are not homogeneously related, then evolution is patently false! But they are.
But is homology also consistent with a designer? We can never be sure, because no one knows if a designer would need (or want) to design homologously. And unfortunately, we cannot falsify homology by design, because no matter how similar or dissimilar two animals may be, or how oddly designed, or how many may go extinct, the creationist can always claim that this is the way God wanted it.
In the end, I think evolution does gain some credibility from homology (alone) simply because it’s consistent with what we’d expect to see if evolution were true. God was certainly under no obligation to design every single animal homologously (especially those created in his image), and it might even be considered somewhat deceptive for him to have done so. Homology by descent can also be falsified, while the nature of homology by design does not allow for potential falsification or even criticism (beyond citing a few seemingly inept designs).